The distribution of Substance P (SP), Somatostatin, Enkephalin and Serotonin (5HT) elements in the developing rat spinal cord will be examined. The initial appearance of these elements within specific areas of the mid-cervical enlargement, the mid-thoracic cord, and the mid-lumbosacral enlargement will be determined by immunohistochemical techniques. Thereafter, their ontogeny will be traced for 12 months. Their morphological and distributional characteristics in this time frame will be examined. These studies give a broad understanding of the anatomical distribution and development of spinal elements implicated in nociception. We will also evaluate the neurotoxic effect and specificity of capsaicin for neonatal SP primary afferents aiming to produce developing animals deprived of SP primary afferents. Information derived from these developmental and experimental studies are essential for future evaluations of the possible developmental plasticity of identified elements that survive the capsaicin treatment.